1. Field of the Invention
The invention relates to a method for controlling cell change in a service network, in which method a terminal performs neighbour cell measurements for cell change, the network makes a decision about the cell change on the basis of the results of said measurements, current network load and the service needs of the terminal, and the network sends to the terminal a cell change order instructing it to switch over to a new cell. The invention additionally relates to a base station of a cellular radio system, which base station comprises means for providing service and means for receiving signalling messages from a terminal as well as means for generating signalling messages and sending them to terminals. Furthermore, the invention relates to a terminal of cellular radio system, which terminal is equipped with means for connecting to a certain service and which comprises means for receiving signalling messages from base stations and means for performing cell specific measurements in order to find a suitable serving cell. In addition, the invention relates to a cellular radio system comprising base stations and associated cells as well as terminals, wherein the base stations are equipped with means for conveying signalling messages between the base station and a terminal, and the terminals are equipped so as to operate at a certain service level and to convey signalling messages between the terminal and base station.
2. Brief Description of Related Developments
To provide background for the invention we will next describe some examples of prior-art solutions for changing the active cell in GSM (Global System for Mobile telecommunications) and in the General Packet Radio Service (GPRS) associated with it. FIG. 1 illustrates the basic structure of a GSM network. The mobile communication network typically comprises at least one core network (CN) and one or more radio access networks (RAN). Core networks comprise various switching systems which may offer different value-added services in addition to the versatile communications features. A core network comprises mobile services switching centres (MSC), other network elements, which in GSM include e.g. the serving GPRS support node (SGSN) and gateway GPRS support node (GGSN), and the associated transmission systems. The radio access networks are located between the core network and the terminals. A radio access network comprises base stations (BS) and a radio network controller (RNC). Each base station is connected via a fixed line to the radio network controller. The radio network controllers in turn are connected via fixed lines to at least one core network node. Between the terminals and core networks there may operate one or more radio access networks, and a terminal may be connected to a plurality of core networks via a radio access network.
A terminal of a cellular radio system always attempts to choose a base station in the coverage area, or cell, of which it camps. In the exemplary situation depicted by FIG. 2, a need for a change of cells arises at least at points P2 to P7. Conventionally, the cell change has been based on measuring the strength of the received radio signal from neighbouring cells 1 to 11 either at the terminal or base station. In GSM, for example, each base station broadcasts, at a so-called broadcast frequency, a signal which has different frequencies for the adjacent base stations. In GSM, a base station sends to a terminal, on the so-called broadcast control channel (BCCH), the parameters used by the terminal to calculate the so-called C values. Most commonly, the so-called C1 value is used in the calculation. The calculation of the C1 value is described in more detail in Michel Mouly, Marie-Bernadette Pautet, “The GSM System for Mobile Communications”, section 7.1.2.2.
Terminals have to measure the reception levels of the received broadcast signals in order to be able to calculate the C1 values of the cells. The cell with the highest C1 value is the most advantageous for the radio connection. In order to optimize cell changes the network may also send additional parameters which make possible to use so-called C2 values. A more detailed description of the use of these parameters is given in reference [1]. The base stations also send to terminals information about the BCCH frequencies used in the adjacent cells so that the terminals know what frequencies they have to listen to in order to find the BCCH broadcasts of the adjacent cells.
GSM system cells provide users with basic services: audio transmission, slow data transmission, and various short message services. GSM operation has been standardized by the European Telecommunication Standardization Institute (ETSI). In GSM, the cells overlap with respect to their coverage areas, so at a cell boundary a decision has to be made about which one of the cells the terminal is to use. In addition to the quality of the connection the decision is based on other factors, too, such as the traffic load of the different cells of the network, for instance. In the active state, the decision about which cell the terminal is to use, as well as the decision about the cell change timing is made by the network, which then informs the terminal. According to an ETSI standard, a terminal in the active state cannot by itself make a decision to start using the service of another cell. A terminal in the idle state makes the decision about which cell it belongs to based on the cells' C1/C2 values which it has calculated. From the terminal's standpoint the moment of cell change is then not of great significance as all the system's cells offer the same basic services and the user of the terminal will not notice the cell change.
FIG. 3 shows as an example a flow diagram of prior-art operation in a cell change situation in connection with a basic GSM service and GPRS service. While in the idle/active state 30 the terminal performs, in the example case depicted by FIG. 3, neighbour cell measurements on the basis of which the network may estimate 31 the need for a cell change. If the result of the estimation is that a cell change is not needed, operation returns to the initial state 30. If a cell change is needed the network chooses in step 32 one of the neighbour cells as the new target cell and sends to the terminal a cell change order 33. This order may be conveyed to the terminal in the basic GSM on the common control channel (CCCH) or Slow Dedicated Control Channel (SDCCH) advantageously using a RR_CELL_CHANGE_ORDER message, in which the new serving cell is described in more detail by means of a base station identity code (BSIC) and at least the information about the absolute radio frequency number (ARFCN) of the new cell's base station. If the terminal at the initial stage 30 is connected to a GPRS service on a cell supporting PCCCH, the cell change order is conveyed on the packet common control channel (PCCCH), advantageously using a PACKET_CELL_CHANGE_ORDER message. Having received one of the above-mentioned messages the terminal must immediately attempt 34 a cell change if the operation takes place in a system according to the prior art. If the terminal does not know the timing information of the new cell, it is not possible to immediately get synchronized to the timing used by the new cell, which is indicated at step 35. In this situation the terminal returns to using the original cell and informs 37 the network that the cell change was unsuccessful using e.g. a PACKET_CELL_CHANGE_FAILURE message if the terminal is connected to a GPRS service. After that the network may give a new cell change order to the terminal, possibly targeted at the same cell. In the worst case, there may be several failed cell change attempts and returns to the original cell before a successful cell change. If, however, the terminal knows the new cell's timing information and is thus able to immediately synchronize to the cell's timing, it starts to use the new cell already at step 34. Succesfull step 35 is then followed by the active/idle state 36.
The prior-art approach described above has many drawbacks. The cell change fails if the terminal does not know the timing information of the new cell. If the cell change fails, a service used by the terminal, which is above the basic service, such as e.g. GPRS, may be disconnected because of the failed cell change. In any case the terminal may have to make several attempts to change cells, which adds to the cell change related signalling between the terminal and the base station in the serving cell.
An object of this invention is to provide a cell change procedure which minimizes unsuccessful cell change attempts to cells with which the operation of the terminal cannot be immediately synchronized at the moment of receiving the cell change order.